1. Field of the Invention
The present invention relates to a hydraulic pressure control system for controlling a speed change mechanism of an automatic transmission hydraulically.
2. Description of Related Art
In a conventional automatic transmission which is often used for vehicles and the like, the hydraulic pressure to be applied to frictional coupling elements is controlled in a changeover fashion by means of a hydraulic pressure control valve. Shift control is performed by a manual operation using a shift lever which is operated by a driver to select one of forward, neutral and reverse gear shift ranges, or an automatic gear shift using an automatic transmission controlling computer, or an electronic control unit which determines a coupled or uncoupled state of friction coupling elements so as to give an appropriate gear ratio in accordance with a throttle valve opening of an engine.
Japanese Patent Application No. Hei 7-264171 proposes a type of a hydraulic pressure control system for an automatic transmission which is provided with a lockup clutch for preventing the occurrence of slip in a torque converter at the time of power transfer from an engine and thereby improving the power transfer efficiency. The hydraulic pressure control system described therein includes, as lockup controller, a changeover valve for changing over oil paths between a release chamber and a locking chamber of a lockup clutch, and a solenoid valve for controlling the hydraulic pressure of the release chamber. Such a lockup controller is usually provided with a gear shift range determining valve which generates a lockup permit signal according to a vehicular gear shift range to operate the changeover valve.
In the above conventional hydraulic pressure control system, however, it is necessary to use a gear shift range determining valve as the lockup controller in addition to a direction change-over valve which is constituted by a spool valve. Consequently, there has been the problem that a hydraulic circuit used in the lockup controller becomes complicated and larger in size.